The best replacement for flint is chert.

The best alternatives to flint are rated as following:
★★★★★  chert
★★★  obsidian
★★★  square
★★  cynthia
★★  bluesky
★  magnesium fire starter
★  matches
★  flynn
★  percussion
★  big sky
★  falwell
★  flint
  jasmine
  lighter
  mini flint
  jerry falwell
  in like flynn
  people
  volkner
  cynthia episode
  magnesium
  supatrem
  blue sky
  borosilicate glass

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Speaking about flint, one probably shouldn't forget about knife.

Other search terms related to flint and their ratings are:
★★★★★ knife
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Google Trends

Scale is based on the average worldwide traffic of flint in all years. Click on graph for more.
flint
chert
obsidian